Maybe because she was doing a bit of all these choices :)

Tales from Under the Sea (released shortly after the film) revealed that Ursula was already queen. But she was evil so Triton deposed her and banished her in a cave of sorts where she spent years trying to escape from. I think this works well with the original concept (plus the Broadway musical) of Ursula and Triton being siblings. In my headcanon, Ursula must have been the oldest sibling so she inherited the throne.